1. Technical Field of the Invention
This invention broadly relates to water used to prepare fluid concrete, which is referred to in the concrete construction art as plastic concrete. The invention further relates to the physical separation of the liquid and solid components of plastic concrete, and, subsequent to the said physical separation, the invention still further relates to the chemical treatment of the liquid component of the plastic concrete to render the liquid component useful as mix water to make new plastic concrete.
2. Description of the Prior Art and Problems Solved
It is well known that concrete is useful as a basic and highly versatile material of construction. The versatility of concrete stems, at least in part, from the fact that concrete initially occurs in a fluid condition, which, after a period of time, converts to a monolithic solid condition. In the fluid condition, concrete, being comprised of discrete solids dispersed in water, has little or no compressive strength; it can be pumped; and it will flow into and assume the shape of a container, referred to in the art as a form. After a period of time, fluid concrete converts to the dense, rigid, monolithic solid condition having the shape of the form and exhibiting substantial compressive strength. For purposes of this disclosure, the fluid condition of concrete, is referred to herein as plastic concrete, and the monolithic solid condition of concrete is referred to herein as set concrete.
The solids dispersed in concrete are comprised of hydraulic cement and aggregate wherein the aggregate ordinarily, but not always, consists of graded coarse aggregate, i.e. rock, mixed with graded fine aggregate, i.e. sand.
It is known that plastic concrete can be prepared in one location, which can be remote from the place of its intended use, and then conveniently transported by known means, such as in a truck, to the place of its intended use. It is also known that plastic concrete can be prepared while being transported to the intended place of use. A truck ordinarily employed to transport plastic concrete is referred to in the art as a ready mix truck.
After plastic concrete has been delivered to the intended location of use in a ready mix truck and caused to flow therefrom into a form, it is known that some plastic concrete may be left in the truck or, at least, that the walls of the truck will be covered with a wet hydraulic cement residue. It is known that any plastic concrete or wet hydraulic cement residue remaining in the truck will conform to the shape of the truck as if it were a form, and, if permitted to remain undisturbed therein for a sufficient period of time, that the cementitious material will hydrate to thereby produce set concrete.
Operators of ready mix trucks do not desire plastic concrete or wet cement residue to set in their trucks. Accordingly, it is well known in the art that operators attempt to remove unused plastic concrete and/or wet hydraulic cement residue, both of which, separately and collectively, are, for convenience, referred to herein as "unset cement", from a ready mix truck by vigorously contacting the unset cement with water to dilute the unset cement whereby it is flushed from the truck. It is apparent that this water flush technique creates a disposal problem. At one time the disposal problem was not addressed, because the wash effluent, consisting of plastic concrete and a highly diluted mixture of aggregate and hydraulic cement, was merely dumped on the ground. This, method can be an environmental problem, and, furthermore, constitutes a waste of water, aggregate and hydraulic cement. The handling and treatment of wash effluent and a solution to the disposal problem is the subject of this disclosure.
The prior art has attempted to harmonize the need to thoroughly wash unset cement from trucks while avoiding the practice of dumping the wash effluent on the ground. In this regard, the art has developed a method for physically separating the component parts of unset cement comprised of the steps of flushing unset cement from a container, such as a ready mix truck, by diluting the unset cement in the ready mix truck with a sufficient quantity of water to produce a wash effluent, comprised of aggregate, hydraulic cement and water, which will flow from the truck; placing the wash effluent in a storage/holding tank; and then performing a further series of steps, referred to in the art as concrete reclaiming, to separate the aggregate from the other named components of the wash effluent.
Concrete reclaiming is known in the art and is broadly comprised of the steps of contacting the mentioned wash effluent with a quantity of water sufficient to physically separate the hydraulic cement component from the aggregate component to form a solid phase which is distinctly separate from the liquid phase; separating the solid (aggregate) phase from the liquid phase in a clarification step or a sieve and size separation step to thereby produce one or more solid phase streams and a liquid stream; and, thereafter, storing the aggregate or aggregates for future use in new plastic concrete and storing the liquid stream, referred to in the art and herein as "gray water," in a holding pond or storage tank.
Gray water is not a true solution. It is comprised of finely divided solids which are dispersed in an aqueous liquid phase as well as solids which are dissolved in the aqueous liquid phase. In the prior art method described above, gray water is placed in a holding pond and is permitted to remain undisturbed therein for a time sufficient for the solid components thereof to settle and for the liquid component thereof to evaporate or to separate whereby the liquid can be removed by pumping. The solids are then excavated from the pond and disposed of in a landfill. A disposal problem, accordingly, still exists and water is still wasted.
The ability to use gray water to make new plastic concrete is a desirable object. If this use of gray water could be realized, then there would be no disposal problem and there would be no waste problem because the aggregate reclaimed could be recycled, as it now is, to make new plastic concrete. and the gray water, which, as defined, includes the liquid component of the reclaimed plastic concrete and wash water, could also be recycled to make new plastic concrete. The prior art, however, as discussed below, has not solved the problem of treating gray water to render it completely useful as mix water to make plastic concrete.
It has been observed that set concrete made with mix water having an unacceptably high concentration of dissolved alkali metal exhibits low durability and tends to crumble. The prior art has not solved this problem in that it has failed to provide a method of treating potential mix water, for example gray water, to convert alkalies dissolved in such water to a form which will not cause the production of set concrete which can be weak and/or unstable. In the context of this invention, the phrase "dissolved alkali metal" and the word "alkalies" refer to sodium, potassium and mixtures thereof which is/are dissolved in the mix water. The sum of the concentrations of dissolved sodium and dissolved potassium is expressed as sodium oxide equivalents, Na.sub.2 O!eq, in parts per million parts (ppm). To this end, the AMERICAN SOCIETY OF TESTING MATERIALS (ASTM) has published a Standard Specification For Ready-Mixed Concrete, ASTM C 94-89b, which establishes set time and performance criteria for plastic concrete which is made with water having questionable or otherwise unknown chemical content, and specifies the concentration limits of certain chemicals present in such water. Tables 1 and 2, below, contain the set time, performance and concentration limits referred to above.
TABLE 1 ______________________________________ ACCEPTANCE CRITERIA FOR QUESTIONABLE WATER SUPPLIES TEST PROPERTY LIMIT METHOD ______________________________________ Minimum Compressive Strength, @ 7 days 90% ASTM C 109 as a percent of control Set Time, deviation from control, Hrs:Mins ASTM C 191 minimum earlier set 1:00 maximum later set 1:30 ______________________________________
TABLE 2 ______________________________________ CHEMICAL LIMITATIONS FOR MIX WASH WATER DISSOLVED CHEMICAL LIMIT maximum concentration in mix water ppm TEST METHOD ______________________________________ Chloride, expressed as C1 ASTM D 512 prestressed concrete 500 concrete in bridge deck 500 concrete in moist environments 1000 concrete containing aluminum 1000 concrete containing dissimilar metals 1000 concrete contacting galvanized metal 1000 SULFATE, expressed as SO.sub.4 3000 ASTM D 516 ALKALIES, expressed as Na.sub.2 O! + 600 0.658K.sub.2 O! TOTAL SOLIDS 50,000 AASHTO T26 ______________________________________
With regard to the data in Table 2, notice that water having an ALKALIES concentration greater than 600 ppm cannot be employed as mix water to make plastic concrete which complies with ASTM specifications. It has been observed that gray water can, and often does, contain "dissolved alkali metal" in excess of 600 ppm and is, thus, not useable as mix water to make plastic concrete meeting ASTM specifications. However, such water, as demonstrated in Example 3, below, can be employed to make concrete, but the water does not comply with ASTM specifications.
This invention, accordingly, provides a method of treating water, for example gray water, containing alkali metal dissolved therein to convert the dissolved alkali metal content thereof to a nonreactive form or forms. The method thus operates to reduce the concentration of alkalies dissolved in the water whereby, for example, the water is rendered useable as mix water which meets ASTM specifications for making plastic concrete. The treated water is usable as mixing water without performing further steps to physically remove the alkali metals therefrom. Thus, in one preferred aspect of this invention, there is provided a method of using all components, solid and liquid, of old plastic concrete, and wash water to make new plastic concrete. This invention, accordingly, avoids the waste disposal and environmental problems associated with methods currently employed.